A traditional light bulb is an example of a lighting device comprising a light source, i.e. the lamp filament, having two terminals for receiving power. When a voltage is applied to such light bulb, a current flows through the filament. The temperature of the filament rises due to Ohmic heating. The filament generates light, having a color temperature related to the temperature of the filament, which may be considered as being a black body. Normally, a lamp has a nominal rating corresponding to a nominal lamp power at nominal lamp voltage, for instance 230 VAC in Europe, and corresponding to a certain nominal color of the emitted light.
Since many decades, people have been used to the light of incandescent lamps of different powers. The light of an incandescent lamp provides a general feeling of well-being. Generally, the lower the power of the incandescent lamp is, the lower the color temperature of the light emitted by the lamp is. As a characterization, the human perception of the light is “warmer” when the color temperature is lower. With one and the same incandescent lamp, the lower the power supplied to the lamp is, which occurs when the lamp is dimmed, the lower the color temperature of the emitted light is.
It is already known that it is possible to dim a lamp, i.e. to reduce the light output. This is done by reducing the average lamp power by reducing the average lamp voltage, for instance by phase cutting. As a result, also the temperature of the filament reduces, and consequently the color of the emitted light changes to a lower color temperature. For instance, in a standard incandescent lamp having 60 W nominal rating, the color temperature is about 2700 K when the lamp is operated at 100% light output while the color temperature is reduced to about 1700 K when the lamp is dimmed to a 4% light output. As is commonly known to a person skilled in the art, the color temperature follows the traditional black body line in a chromaticity diagram. A lower color temperature corresponds to a more reddish impression, and this is associated with a warmer, more cozy and pleasant atmosphere.
A relatively recent tendency is to replace incandescent light sources by lighting devices based on LED light sources, in view of the fact that LEDs are more efficient in converting electric energy to light and have a longer lifetime. Such lighting device comprises, apart from the actual LED light source(s), a driver that receives the mains voltage intended to operate an incandescent lamp and converts the input mains voltage to an operating LED current. LEDs are designed to provide a nominal light output when operated with a constant current having a nominal magnitude. An LED can also be dimmed. This can be done by reducing the current magnitude, but this typically results in a change of the color of the light output. In order to keep the color temperature of the generated light as constant as possible, dimming an LED is typically done by Pulse Width Modulation, also indicated as duty cycle dimming, wherein the LED current is switched ON and OFF at a relatively high frequency, wherein the current magnitude in the ON periods is equal to the nominal design magnitude, and wherein the ratio between ON time and switching period determines the light output.
It is desirable to have a lighting device having one or more LEDs as light source, wherein the dimming behavior of the traditional incandescent lamp is simulated so that, on dimming, the color temperature of the output light also follows a path (preferably close to the black body line) from a higher color temperature to a lower temperature.
Lighting devices capable of such functionality have already been proposed, for instance in US-2006/0273331. Such prior art devices comprise at least two LEDs of mutually different colors, each provided with a corresponding current source, and an intelligent control device, such as a microprocessor, controlling the individual current sources to change the relative light outputs of the respective LEDs. The known device receives an input voltage signal that carries power and a control signal. In the device, the control signal is taken from the input signal and transferred to the intelligent control device, that controls the individual current sources on the basis of the received control data. By changing the ratio between the respective light outputs, the relative contributions to the overall light output is changed and hence the overall color of the overall light output, as perceived by an observer, is changed. Such lighting device, therefore, requires a separate control input signal.
In LED lighting devices, a behavior of the color temperature of the LED light can be obtained which, in dimming conditions, is similar to that of an incandescent lamp, but until now only at the expense of extensive current control, such as e.g. known from DE10230105. The necessity of adding controls to the LED lighting device for the desired color temperature behavior increases the number of components, increases the complexity of the lighting device, and increases costs. These effects are undesirable.